Several different feedback or echo problems can occur in a telephone system comprising two geographically separated handsets with a network comprising a transmission line between them. A signal loop comprising acoustic as well as electric transmission paths from one station to the other and back may be established via feedback from loudspeaker to microphone of each handset, so that e.g. the speech from a first talker at one end picked up by the microphone of the handset and transmitted to a handset at the other end, where acoustic feedback from the loudspeaker to the microphone of the handset occurs, may result in a component of the speech of the first talker returns to the loudspeaker of the first talker's handset as an echo. This situation is schematically illustrated in FIG. 1 (compare original speech element 1: Hello with echo element 6: Hello). Other echo problems may, alternatively, be of purely electric origin due to transmission line reflections or equivalent.
U.S. Pat. No. 3,622,714 describes the use of complementary comb filters in telephonic transmission to minimize such echo problems. FIG. 3 of U.S. Pat. No. 3,622,714 illustrates a system comprising telephone stations (C, D) each comprising complementary filters (30, 31) and (32, 33), respectively. The complementary filters used in the receive parts (comprising speakers 35, 36, respectively) and the transmit parts (comprising microphones 34, 37, respectively) of each station are illustrated in FIG. 2 of U.S. Pat. No. 3,622,714. Filters f15-f25 and f16-f26 each represent a comb filter with a passband width and a center-frequency spacing of each passband of one third octave (i.e. a non-uniform width of the frequency bands).